The present invention generally relates to optical fiber communications and, more particularly, to a heterodyne or homodyne detection/reception system which is suitable for selective reception of optical FDM (frequency division multiplex) signals.
A coherent optical frequency division multiplex communication system has been suggested as a means for providing in the future long-distance and large-capacity communication, since such a system enables high-density optical FDM broadcasting and remarkable optical reception sensitivity when compared with a conventional light-intensity direct-detection communication system for modulating and detecting the intensity of light from a light source.
A conventional system for selectively receiving one channel from an optical multi-channel FDM signal, for example, a system entitled "COHERENT OPTICAL CATV, A 10-CHANNEL FOR TRANSMISSION EXPERIMENT", Shibutani et al. is disclosed in "Institute of Electronics, Information and Communication Engineers", OQE 88-70, pp 45-52, 1988.
FIG. 2 shows an optical channel selection section 40 in a prior art system, in which, a signal 201 is supplied to a frequency discriminator 15 so that an optical frequency of an optical signal 202 emitted from a local oscillation light source 6 is controlled by a random access controller 17 through the frequency discriminator 15, a sweeper 16 and a channel selector 18. Shown in FIGS. 3A, 3B and 3C are graphs showing relationships between local oscillation optical frequencies and optical frequency control current of the local oscillation light source 6 (which current will be referred to merely as the control current) with respect to optical frequency channel positions set at the transmitter side.
More specifically, FIG. 3A shows the relationship between the control current and the optical frequency at the time of starting the system. First, the control current or optical frequency of the local oscillation light source 6 is swept by the sweeper 16 so that channel positions A to D are stored in the form of values a to d of the control current producing light beat.
The random access controller 17 usually automatically controls the frequency of the control current (performs so-called automatic frequency control (AFC)) on the basis of a signal received from the frequency discriminator 15 in such a manner that the difference in optical frequency between the local oscillation light source 6 and selected channel is constant. The random access controller 17, when receiving a channel request signal from the channel selector 18, stops its AFC operation, shifts the control current to a level corresponding to a desired channel stored at the time of starting the system, seizes the closest one of the channels corresponding to the shifted control current value, and again switches to its usual AFC operation.
FIG. 3B shows the relationship between the control current and optical frequency when channel shift is carried out from the selected channel A to a desired channel D. In this case, the random access controller 17 shifts the value a of the control current corresponding to the selected channel A to the value d thereof stored for the desired channel D, and selects the closest one (in this example, channel D) of the channels corresponding to the shifted value.
FIG. 3C shows a state in which optical frequencies of transmission and local oscillation light are shifted from A, B, C and D to A', B', C' and D'. In the prior art system, when channel selection is carried out substantially in the same manner as in FIG. 3B, even shift of the control current to its previously stored value d under the random access controller 17 causes erroneous selection of not the channel D but of the closest channel C'.
Even under a condition such that the optical frequency is not varied, the system is caused to be erroneously operated under the influence of variations in the characteristics of the local oscillation light source, or the like, with time during its continuous operation.
In the prior art system, in order to avoid such erroneous operation as explained in connection with FIG. 3C, it has been necessary to restrictively set variations in the frequencies of transmission and local oscillation light within a range determined by channel spacing.